Frequency modulated wave receiver circuits



Feb'. 26, 1946. of B. HANsoN ErAL 2,395,738 y FREQUENCY MODULATED WAVE RECEIVER CIRCUTS 2 shams-suegray 1 Filed April 1'7. 1940v m A m M H B. R m 0 .IVI

F6525 1946 o B. HANsoN- ET Al. n 2,395,738

l v FREQUENCY MODULATED WAVE RECEIVER CIRCUITS Filed April 17. 1940 f 2 sheets-sheet 2` -ruEs REcr .67

To con/mau Eo k70H55" INV EN TORS asc/m E. HANso/v AND BY RosERT E SHELBY /wv-f/V ATTORNEY.

Patented Feb. 26, 1946 FREQUENCY MODULATED WAVE RECEIVER CIRCUITS Oscar B. Hanson,Y Westport, Conn., and Robert E.

Shelby,;Teaneck, N. 3.,' assignors to Radio Corporation of America, a corporation of Delaware Application april 17, 1940, serial No. 329,998

6 Claims.

Our present invention relates to frequency modulaled carrier Wave receiving systems, and more particularly to arrangements for automatically controlling the pass band width of such systems. n

As is Well known, the pass band width of llthe intermediate frequency (I, F. hereinafter) stages of a receiver of' frequency modulated` carrier waves is a function of the instantaneous ampli.y

tude of the modulating signal. The band width varies from a minimum equal to twice the high; est modulation frequency to any arbitrary maxi-A mum. Grdinarily the pass band width of a frequency modulation (briefly referred to in the following description as FM) receiver isxed at a value sufficiently greatito accommodate all important sidebands generated by the modulating signal at its maximum amplitude. This fixed band width determines thewamplitude of interference in the pre-detectorcircuits, if the interference is of the kind having frequency components distributed throughout va band at least as wide as that of the receiver.y

One of the main objects ofpur present invention, then, is to utilize in an FM receiver a method of, and means for, varying the prei-detector pass band width in accordance with-the amplitude of the modulating signal, the control being so adjusted that the instantaneous passband Width is just slightly greater than necessaryto accommodate the desired signal spectrum.

Another important object of our invention is to provide, in an FM receiver, an arrangement for augmenting the inherent noise discrimination property of the receiver, there being utilized for such purpose a device for varying the effective pass band width of at least one pre-limiter stage in direct rel-ation with the degree of modulation of the carrier wave whereby for low level modulation, or small deviation of the carrier wave' frequency, the pass band would be reduced to a width slightly greater than that required to pass that degree of deviation.

Another object of the invention is to provide an automatic pass band width control device for an FM receiver, wherein the pass band is greaterr than the degree of amplitude represented by the deviation in order that an increase in modulation, in other words an increase in deviation, would reach the detector circuit and cause it to expand the band to accept the demand for a greater b-and width.

Another object of our invention is to provide an automatic pre-limiter pass band control for an FM receiver, wherein the transmitted wave energy embodies a pilot tone, which is amplitude modul-ated on the carrier wave, or is a frequency modulation component, for providing the actuatingV energy for the automatic band control arrangement.

Still other objects are to improve generally `the l eiciency and reliability 'of FM receivers, and Y more' especially to provide bandWidthj-control arrangements for' FM receivers wh li nomically assembled in the receiver?.L4 e The novel features which webelieve o be characteristic of our invention are set forthwith particularity in the appended claims; the `inven-A tion itself, however, as to both its organization .y

and method of operation will best be understood by reference to the following description taken in connection with the drawings in which we have indicated diagrammatically several circuit organizations whereby our invention may be carried intoeffect.

lIn the drawings: l

Fig. 1 shows an FM receiver embodying the invention,

Fig. 2 schematically shows a modification, wherein the FM wave embodiesa frequency modulation pilot tone,

Fig. 3 shows a modification of the arrangement in Fig. `2, wherein the pilot tone is amplitude modulated on the FM carrier.

Referring now to the drawings, wherein like reference characters in the different figures designate similar circuit elements, there will first be described the general'stages embodied in a typical FM receiver. The signal collector I may be of any well known type such as adipole,` iflth'esig'- nal frequency range is in the'43 megacycleV (mc.) range, or it may be of the usual Vertical antenna type, and, if desired, the collector may be a radio frequency distribution line; The collected signals are impressed upon a converter stage 2, and usv ually there will be employed one or more tunable'l radio frequency amplifier stages. The converter may be of the combined local oscillator-first detector type utilizing a pentagrid converter tube of the 6A? type. However, separate first detector and local oscillator tubes may be employed. Regardless of the construction of the networks up to the I. F. stages it will be understood that the numeral 3 denotes the tunable local oscillator tank circuit and the numeral ldenotes the signal circuit tuning instrumentality. The usual unicontrol mechanism is employed for varying the signal circuit condensers as well as the oscillator tank circuitv condenser, and is designated as Tuner. f

The I. F. voltage developed by the converter is transmitted to one or more I, F. ampliers. One of the amplifiers is schematically represented by numeral 5, Whereas the following I. F. amplifier is shown in detail in order to illustrate the manner in which the invention is utilized in conjunction with the I, F. amplifier. The second I. F. amplier tube 6 has its input electrodes coupled to the resonant secondary circuit of the I. F. transformer '1, and it will be understood that each of the primary and secondary circuits of transformer 'I are Xedly resonated to the I. F. value, The output electrodes of tube 6 are connected to the primary circuit of the following I. F. trans..

former 8, and each of the primary and secondary circuits of transformer 8 is fixedly resonant to the operating I. F. value.

If the receiver is constructed to receive modulated carrier signals in the 43 mc. range, then an I. F. value of some 2.2 mc. is best utilized. With the latter I. F. value is'desirably utilized a normal I. F. bandwidth of some 200 kilocycles (kc.) Of course, the invention is not limited to wide band FM reception, since it may be employed with equal facility in narrow band reception wherein the I. F. band width is of the order of 40 to 75 kc. It is pointed out that at the present time rthere are some ve channels allocated to the 43 mc. range, each of the channels being spaced 0.2 mc. apart.

It is to be understood that I. F. amplifier stages and 6 will be constructed in a similar fashion, and, therefore, the following description relating to the second I. F. amplier tube 6 holds equally true of the amplier 5. Tube 6 has its cathode connected to ground through the usual self-bias resistor 9', the latter being shunted by an I. F. bypass condenser. Proper energizing potential for the plate of tube 6 may be derived from any well known source of direct current.

4 The stage following the I. F. amplifier 6 is an amplitude modulation limiter 9, and there is shown immediately abo've the rectangle labelled AM Limiter a qualitative representation of the limiter characteristic. skilled in the art, the limiter, regardless of its construction, must have a characteristic such that the input and output voltages are directly proportional up to a predetermined signal input amplitude, and thereafter the characteristic substantially flattens out so that no further output results from an increase in signal input voltage amplitude. In this way any amplitude modulation which appears on the FM carrier wave is substantially eliminated, and does not appear at 55 the input circuitof the following detector stage I0. It is not believed necessary to describe the limiter circuit in any further detail since those skilled in the art are fully aware of its construction, and know that the tuned input circuit 8' 60 thereof has applied to it FM energy at the I. F. value, and that in the output circuit of the limiter there is developed L F. carrier voltage which is substantially purely frequency modulated.

The FM detector I0 is also Wellknown in the 65 art, and there is graphically represented above the rectangle labelled FM detector the characteristic of the detector network. Regardless of the construction of the detector, the latter is constructed to deliver to the audio amplifier a 70 uni-directional voltage whose magnitude is proportional to the frequency deviation of the I. F. voltage from the predetermined I. F. value. The detector generally comprises a pair of rectifiers As is well known to those o varying device.

input circuit which may comprise a pair of resonant circuits oppositely and equally mistuned from the predetermined I. F. value. However, the input circuit, or discriminator circuit, may be of the type utilized in modern automatic fre quency control systems wherein the discriminator comprises cascaded primary and secondary circuits each tuned to the I. F. value, but wherein the high potential side of the primary is connected to a mid-tap on the secondary. In either discriminator construction the FM wave is converted to an amplitude modulated wave which is impressed upon the opposed rectiiiers.

The detector output voltage consists of the sum of two voltages, one from each rectifier. Reference is made to the application of D. E. Foster and J. A. Rankin, application Serial No. 319,830, led February 20, 1940, granted Feb. 17, 1942, as U. S. Patent No. 2,273,097, for a disclosure of a specific type of limiter and FM detector which may be utilized at stages 9 and I0 of this application. YThe audio voltage output of the detector I0 is impressed upon one or more audio ampliners II, and the latter may be equipped with an audio gain control device schematically represented by numeral I2. The output of the' audio amplilier I I may be impressed upon any utilizing network which terminates in a reproducer such asa loudspeaker.

Instead of being proportional to the strength of the received carrier, it is desired in the present case that the control voltage for the I. F. band Width control network be proportional to the amplitude of the modulation on the carrier. Within the dotted rectangle I3, which represents the control voltage source, there is illustrated a circuit for deriving from the audio modulation voltage a uni-directional control voltage which can be utilized to control the I.V F. band width The control circuit shown within the rectangle I3 has been disclosed by J. L. Hathaway in application Serial No. 232,273, filed Sept. 29, 1938, granted June 10, 1941, as U. S. Patent No. 2,244,695. In the latter patent there is disclosed acircuit wherein the audio voltage is impressed upon an amplier, and the amplified audio voltage is then applied to a pair of diode rectiers of the delayed type, the rectiers being arranged to function as a full wave rectifier network.

Specically, the amplier tube I4 has its cathode grounded, while itsV control grid is connected to a resistor I 5 having its upper terminal connected by a direct current blocking condenser I6 to the high potential side of the audio output circuit. The low potentialterminal of resistor I5'V Yis connected to any desired negative bias source. and the adjustable Contact element I'I may be varied along resistor l5 so as to adjust the audio input to the amplifier I4. The plate of amplier tube I4 is connected to the positive terminal of the direct current source through the primary coil of the audio transformer I8.

The secondary winding of transformer I8 has its opposite terminals connected to the opposed diode anodes I9 and 20 of the double diode tube 2l. The mid-point of the secondary of transformer I8 is Yconnected to a source of negative bias, and-ifdesired, this bias source may be the same as the bias sourcev for the grid of tube I4. The common cathode of tube 2I is connected to ground through a resistor R, and a condenser C is shunted across resistor R. Across resistor R there is developed the direct current voltage arranged in push-pull relation, and havingan ,which is derived from the rectied, amplified audiovoltage impressed upon the push-pull, connected diodes. l TheY time constant ofnetwork RC is so chosen that control voltage is developed in an increasing sense across resistor R at a relatively more rapid rate than in the reverse sense.

Inother words, thetimeconstant of the full wave rectifier is chosen sok that the control network has the capability for extremely rapid action in the direction of expanded If. F. band width to prevent perceptible audio frequency distortion,

and a much slower action in the direction of nar# rowedl band Width.

As.` explainedin the aforesaid Hathaway Patent, rectification cannot occur until the audio voltage peaks at the diode anodes I9 and 20 exceed the initial negative bias applied thereto. The primary winding of transformerV I8 may have an impedance of approximately 12,000 ohms, while the secondary winding may have an. impedance of 5,000V ohms. The magnitude of condenser C maybe chosen from a range of 0.25 to 1.0 microfarads, While resistor R may have a Value of 2 to 5 megohms. In such case the rectifier charging time, which corresponds to an expanding I. F. band width, will be approximately 0.001 second. The discharge time, which corresponds to acontracting I. F. band width, will be approximately 3 seconds. Of course, these constants and magnitudes are purely illustrative, and it is to be understood that other values are within the scope of this invention depending on the conditions of reception, band width in the I. F. network, and thelike.

Because of the initial bias, rectication starts at a definite input level, and a further increase of input level causes an extremely rapidv bias increase. The relative rate of bias increase with input level increase is a direct function of the initial delay bias applied tothe diode anodes. The control voltage, or bias, across resistor R increases very suddenly when the audio signal applied to the diodes reaches a predetermined amplitude. The chargingA resistance of the circuit is the sum of the diode resistance and the section of the transformer secondary winding which supplies the audio. peak. The discharge time` following an audio peak is determined by the time constant of the condenser C and the resistor R. This time constant isy many times greater than the charging time constant. The reduction of control voltage across resistor R, therefore, is allowed to. proceed ata relatively slow rate, whereas the control voltagey develops in an increasing sense ata relatively much faster rate.

The voltage itself is applied to the band width control tube 22, and the control voltage developed across resistor R functions to regulate the gain, or transconductance, of tube 22. Tube 22A itself is included in a feedback circuit. That is to say. the input gridl 30 of tube- 22 is magnetically coupled through coil 3l to the primary circuit of I. F. transformer 8. Coil 3| is included in the direct current voltage connection 32 to the cathode end of resistor R. The connection 32, which is the control voltage connection, includes a filter resistor 33, shunted to ground by the condenser 34, and which functions to suppress audio pulsations in the control voltage. The cathode of tube 22 is connected to ground through a self-bias resistor 40. Hence, in the absence of received signalenergy, the voltage developed across resistor 40 provides the initial negative bias for the grid 30.

Thegrplate 4 I- of tube 22 is connected to the positiveterminal of a direct current source through the coil; 42, and thelatter is magnetically coupled to the secondary circuit of the I. F; transformer. 1. 'Ihe tube 22 will, therefore, be seen tohave its input electrodesmagneticallycoupled to the tuned output circuit off I. F. amplier 6l, while its output electrodes arev magnetically coupled to the tuned'input circuit. of amplifier E. The-feed.'- back coupling circuitsare so poled that-they pro.- duce degeneration atl the I. F. carrier frequency, that is at the predetermined I. F. value of the input and, output circuits of'amplier 6. Con-` versely, regeneration is produced at frequencies near the opposite edges of the I. F. pass. band; Hence, when the transconductance of the feedback tube 22 is a maximum, the pass. bandwidth is of maximum value by virtue of the aforedescribed feedback effects. Reference is made to a paper entitled Receiver with automatic selecti-v` ity control responsive to interference,` by J. F; Farrington", published in the Proceedings of the Institute of Radio Engineers for April, 1939', for a more detailed disclosure of the above type of feedback circuit.

It has been shown in the last namedy publication that a feedback circuit of the type disclosed herein in connection with tube 22 may have its gain varied', thereby to vary the effective band width of the amplifier having the feedback circuit arranged between its output and input circuits. Of course, if the maximum bandwidth at the I. F: carrier is to be of the order of 200 kc., then the feedback circuit is so arranged that the feedback through the tube 22 is a maximum for the maximum control voltage developed across resistor R. It will, therefore, be seen that when the audio modulation magnitude is a maximum the con' trol voltage across R is a maximum, and the latter voltage produces maximum gain of tube 22.. In turn, the feedback through tube 22 is a maximum, and hence the effective I. F. band Width is also a maximum.

The signal grid of amplifier 6 is connected through the controlv connection 32", which inf cludes the resistor 5B, to the lead 32. Inthisway the gain of tube 6 will also be a maximum Iwhen the feedback 22 causes a wide I. F. band width to be produced. Accordingly, any decrease in I. F. signal voltage magnitude caused by the feedback tube, will be compensated for by the increased gain of amplifier tube 6. It will now be seenthat the effective band Width of the` I. F. amplifier 6 is a function of the audio modulation magnitude,l and that the expansion of the I. F. band width proceeds at a faster rate than the narrowing of, the band width by virtue of the type of control. circuit utilized.

It is desirable to keep the `band width of the I. F. circuit always just slightly greater than necessary to accommodate the desired signal spectrum at any instant. The. advantage of such an arrangement will at once be apparent, particularly for cases where the amplitude of the interference is `close to the threshold value say, for example, 6 decibels below, the desired signal. However, it is to be` clearly understood that instead of employing the specific type of feedback circuit shown for controlling the band width, there may be utilized in place thereof damping tubes, connected across the successive I. F. resonant circuits, in a manner disclosed by G. L. Beers in a paper entitled Automatic selectivity control, published inthe Proceedings of the Institute of Radio Engineers inv December, 1935, page 1425'.

ducing sense.

Y cuit may be employed in the receiver by coupling any desired type of rectifier 5| to the input of the limiter, and applying the resulting control voltage of the rectifier to one or more of the -preceding signal transmission tubes in a gain re- As is Well known to those skilled in the art, the A. V. C. circuit functions to vary the gain of the early tubes in a manner to maintain the carrier amplitude to the limiter input circuit substantially uniform regardless of carrier amplitude variation at the signal collector device.

The advantage of the present invention will now be realized. Since in an FM wave the audio modulation appears in the wave asV a varying carrier frequency, then the receiver of necessity must receive and detect waves of variable band Width. In order to avoid increase in interference I`and increase in noise reproduction when the audio modulation causes the received wave to be of relatively narrow band Width, the control circuit herein functions automatically to narrow the I. F. pass band width to accommodate the instantaneousI FM wave band Width and thereby exclude interference or noise components. In other words, the effective I. F. band width of amplifier 6, and it is to be understood that this may also be repeated in connection with amplifier 5, is a direct function of the amplitude of the modulation signal on the carrier wave. In FM transmission the carrier wave is varied in frequency in accordance with the modulation signals; the extent of deviation of the frequency relative to the mean or carrier frequency is a function of the modulation signal amplitude, while the rate of deviation depends on the modulation signal frequencies per se. A phase modulated carrier wave differs from an FM wave essentially in that the frequency of the carrier Wave is deviated to a relatively greater extent at the higher modulation signal frequencies. In other words, a phase modulated Wave is similar to an FM wave except that there exists pre-emphasis for the higher modulation signal frequencies.

It is, of course, possible in accordance with our invention automatically to vary the band width of the I. F. network in response to a pilot tone transmitted ralong with the FM wave-to the receiver. For example, in Fig. 2 there is shown schematically an arrangement wherein there is filtered out of the audio amplifier network, by a pilot tone ilter 60, a frequency modulation component which may be a super-audible tone. The super-audible tone energy is impressed on a rectier 6|, and the rectied voltage may be utilized in the manner shown in Fig. l` for controlling theV gain of the feedback tube 22. Those skilled in the art are fully aware of the manner of including a super-audible tone in the main modulation of the-carrier wave. By making the pilot tone super-audible it is prevented from interfering with the desired program modulation.

The pilot tone need not be a frequency modulation component of the FM wave, but may, if desired, be a tone which is amplitude modulated on the carrier at the transmitter along with the main frequency modulation. Such a pilot tone would be proportional to the instantaneous amplitude of the modulating signal. In such case the receiver would utilize an amplitude modulation detector 10 having its input coupled to the input of limiter 9. The detected output of network l!) would then be transmitted through a pilot tone filter 'l I, and the filtered tone energy would then be rectied fby a rectifier 12. The output of rectifier 12 would b'e employed in the same manner as the control voltage in Fig. 1 to control the feedback tube 22. Since in this case the amplitude modulation component on the carrier is proportional to the instantaneous amplitude of the main modulating signal, the output of rectifier 'l2 would be of the correct polarity and magnitude for the control of the I. F. band width.

While we have indicated and described several systems for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of our invention, as set forth in the appended claims.

What we claim is:

1. In a frequency modulation receiver provided with at least one intermediate frequency amplifier and a frequency modulation detector, a network coupled to the input of said amplifier for delivering thereto frequency modulated waves including a pilot tone as a, component thereof, means for separating said tone component from the waves, meansfor rectifying the said pilot tone component, and means for varying the effective band width of said amplifier with the rectified tone component.

2. In a frequency modulation receiver provided with at least one intermediate frequency amplifier and a frequency modulation detector, a network coupled to the input of said amplier for delivering thereto frequency modulated waves including a pilot tone as a component thereof, means for separating said tone component from the waves, means for rectifying the said pilot tone component, means for varying the effective band Width of said amplifier with the rectified tone component, said pilot tone being an amplitude modulated component of the carrier wave, and its magnitude being proportional to the amplitude of the modulation on the carrier Wave.

3. A method of signalling which comprises receiving a modulated carrier wave whose frequency is a function of the modulation of the carrier, and which wave has a characteristic thereof variable in accordance with the magnitude of a predetermined pilot frequency Wave component, deriving a voltage from the received wave which is dependent upon solely the magnitude of said pilot, and varying the selectivity of reception of said modulated wave in response to the magnitude of saidl derived voltage in a predetermined manner.

4. A method of receiving a frequency modulated carrier wave which has as a modulation component a super-audible tone, comprising demodulating the wave to produce the modulation components, deriving from solely the superaudible modulation component a control voltage, and varying the selectivity of transmission of the modulated wave energy prior to said demodulation with said control voltage.

5. A method of receiving a carrier wave which has been varied in frequency by the desired modulation and which has been varied in amplitude by a predetermined pilot tone, comprising subjecting the modulated carrier energy to demodulation subsequent to amplitude limiting, subjecting the modulated carrier energy to amplitude demodulation prior to said limiting to produce said pilot energy, deriving a control voltage from. the pilot energy, and varying the selectivity of transmission of said modulated carrier energy with said control voltage prior to said limiting.

6. A method of operating a receiver of frequency modulated carrier Waves of the type comprising a demodulator preceded by a selective transmission circuit; the steps of impressing on the receiver said Waves, including a pilot tone in said Waves, prior to said impression, as a component thereof, passing the waves through said selective circuit, demodulating said passed waves, separating the pilot tone, rectifying the pilot tone, and regulating the selectivity of said 5 transmission circuit With rectified pilot voltage.

OSCAR B. HANSON. ROBERT E. SHELBY. 

